Commercial airplanes and components thereof are increasingly being manufactured from composite materials that may require ultrasonic inspection to determine part integrity. Inspections may be performed from one side (i.e., pulse-echo inspections) or from two sides, referred to herein as the through-transmission (TTU) methods. In either case, a transducer that transmits and/or receives sound waves must be acoustically coupled to the inspected part. Water or some other liquid is typically disposed between the transducer and the inspected part to provide a medium through which sound waves can travel. This can be accomplished by submerging the transducer and the part within a pool of water during inspection, but this method is inefficient and not practical for large parts.
Alternatively, a method known in the art as the “thin film” method may include a shoe surrounding the transducer and interfacing with the part to at least partially contain the water locally between the transducer and the part. However, because water leaks out between the shoe and the part, new water must be continuously pumped in to replace the water escaping. This method also requires that the escaping water be captured. Furthermore, when the surface of the inspected part is sharply contoured or roughly textured, the leakage rate may become so high that the water cannot be adequately contained long enough to maintain coupling.
In some alternative methods, a conformable solid or gel couplant is used to fill a gap between the part to be inspected and the transducer. For example, the gel may be Aqualene material by Olympus or Aquaflex by Parker Labs. However, these solid or gel couplants are not suitable for C-Scan inspections of sharply contoured or roughly textured composite parts, because they are too rigid to conform well to roughly textured surfaces and are easily damaged by sliding over roughly textured surfaces.